1. Technical Field
The present disclosure generally relates to radio frequency (RF) circuit components, and more particularly, the quasi-differential RF power amplifiers with a high level of harmonics rejection.
2. Related Art
An RF communications system is generally comprised of a transmitter that generates signals that are radiated as radio frequency electromagnetic waves by an antenna, and a receiver that converts the electromagnetic waves generated by a counterpart remote transmitter and detected by the antenna to usable signal. Due to the transmitter circuitry lacking sufficient output power, the transmitter is typically connected to additional components that are dedicated to amplifying the generated signal, referred to as power amplifiers. Similarly, because the receiver circuitry typically lacks sufficient reception sensitivity, there are additional components also dedicated to amplifying the received signal, referred to as low noise amplifiers.
Certain limitations in amplifier circuits/transistors dictate the use of multiple transistors with outputs thereof being connected to a single load (antenna). There are also circumstances where signals multiple antennas are amplified by a single amplifier circuit. Both of these applications involve a power combiner, and there are a variety of modalities thereof that are known in the art.
One such modality is in-phase power combining, where multiple signals of identical phase and amplitude are combined into a single output, typically at the final stage before connecting to the antenna. There are deficiencies, however, relating to the load voltage standing wave ratio (VSWR) variation being equally applied to different transistors, thus limiting applicability. In particular, high voltage swings across different transistor nodes may become unacceptably high.
Another modality known in the art is quadrature combining. In one variation, identical balanced power amplifiers are connected to input quadrature directional coupler as well as an output quadrature directional coupler in the reverse. Although load VSWR variation has a small influence on the reliability of the transistors, current consumption is fairly high, and an additional harmonics rejection filter is needed in order to pass regulatory requirements. Such harmonics filtering is needed for in-phase power combining circuits as well.
Yet another modality is out-of-phase power combining, where differential signals are equally amplified and combined at an output balun. This technique is understood to relax voltage swing requirements for each transistor in the circuit. Additionally, reliability is improved, and adequate levels of even harmonics rejection can be realized. Existing out-of-phase power combining circuitry, however, are still deficient with respect to odd harmonics rejection at the circuit output. Moreover, optimal performance demands a symmetrical layout or routing of the differential circuit is necessary, along with fabrication on an appropriate semiconductor or other laminate substrate. In order to achieve an adequate level of even harmonics rejection, the input signal imbalance must be kept to a minimal level.
Accordingly, there is a need in the art for an improved out-of-phase power combining modality that addresses the aforementioned limitations of existing implementations. There is also a need for quasi-differential RF power amplifier with a high level of both even and odd harmonics rejection.